Characterization and field emission characteristics of carbon nanotubes modified by titanium carbide

Carbon nanotubes (CNTs) were modified by depositing a thin layer of titanium film on the surface using magnetron sputtering method, followed by vacuum annealing at 900 °C for 2 h. X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) confirmed that the as-deposited thin titanium film reacted with carbon atoms to form titanium carbide after annealing. The experiment results show that the thickness of sputter-deposited titanium film has significant effect on the field emission J-E characteristic of modified CNTs film. The titanium carbide-modified CNTs film obtained by controlling the titanium sputtering time to 2 min showed an improved field emission characteristics with a significant reduction in the turn-on electric field and an obvious increase in the emission current density as well as an improvement in emission stability. The improvement of field emission characteristics achieved is attributed to the low work function and good resistance to ion bombardment of titanium carbide.     

钛碳化物改性碳纳米管的场发射性能

通过在碳纳米管(CNTs)表面沉积钛薄膜并经过高温真空退火处理,在CNTs表面形成了低功函数的钛碳化物.研究了钛碳化物改性CNTs的场发射性能,并利用X射线光电子能谱(XPS)对改性碳管进行了结构表征.实验结果表明,高温真空退火可使沉积在CNTs表面的钛原子与碳原子发生化学反应生成钛碳化物;经钛碳化物改性处理的CNTs的场发射性能明显改善,开启电场由改性前的121降低到104V/μm,当电场强度为234V/μm时,场发射电流密度由改性前的23增大到改性后的13.5mA/cm²,同时,CNTs的表面抗离子轰击能力增强,发射稳定性改善.对钛碳化物改性增强CNTs薄膜场发射性能的机理进行了分析. 关键词： 碳纳米管 钛碳化物 场发射 结构表征     

Preparation and field emission properties of carbon nanotubes cold cathode using melting Ag nano-particles as binder

A new preparation process for carbon nanotubes (CNTs) cold cathode was studied through the replacement of traditional organic or inorganic binder with Ag nano-particles. This method has the advantages of low preparation temperature and fine electrical contact between CNTs paste and substrate. A mixture paste of CNTs, Ag nano-particles and other organic solvents was spreaded on Si substrate. By melting and connecting of Ag nano-particles after sintered 30 min at 250 °C, a flat CNTs films with good field emission properties was obtained. The measurements reveal that the turn on electric field and the threshold electric field of as-prepared CNTs cathode are 2.1 and 3.9 V/μm respectively and the field emission current density is up to 41 mA/cm² at an applied electric field of 4.7 V/μm.     

Study of electronic structures for Fe thin films deposited on Si- and C-faces of 4H-SiC substrates by soft X-ray emission spectroscopy

Silicon carbide (SiC) is a candidate material for electronic devices to operate upon crucial environment. Electronic states of silicides and/or carbide/graphite formed in metal/SiC contact system are fundamentally important from the viewpoint of device performance.We study interface electronic structure of iron thin film deposited on silicon (Si)- and carbon (C)-face of 4H-SiC(0 0 0 1) by using a soft X-ray emission spectroscopy (SXES). For specimens of Fe (50 nm)/4H-SiC (substrate) contact systems annealed at 700 and 900 °C, the Si L_2,3 emission spectra indicate different shapes and peak energies from the substrate depending on thermal-treated temperature. The product of materials such as silicides is suggested. Further, from comparison of Si L_2,3 emission spectra between Si- and C-face for the same annealing temperature at 700 °C, it is concluded that the similar silicides and/or ternary materials are formed on the two surfaces. However for those of 900 °C, the film on substrate is composed of the different silicide and/or ternary materials.     

Study of nickel silicide formation on Si(1 1 0) substrate

Nickel silicide formation on Si(1 1 0) and Si(1 0 0) substrate was investigated in this paper. It is confirmed that nickel monosilicide (NiSi) starts to form after 450 °C annealing for Si(1 0 0) substrate, but a higher annealing temperature is required for NiSi formation on Si(1 1 0) substrate, which is demonstrated by X-ray diffraction (XRD) and Raman scattering spectroscopy. The higher formation temperature of NiSi is attributed to the larger Ni₂Si grain size formed on Si(1 1 0) substrate. Ni silicided Schottky contacts on both Si(1 0 0) and Si(1 1 0) substrates were also fabricated for electrical characteristics evaluation. It clearly reveals that the rectifying characteristics of NiSi/n-Si(1 1 0) Schottky contacts is inferior to that of NiSi/n-Si(1 0 0) Schottky contacts, which is attributed to a lower Schottky barrier height and a rougher contact interface. The formation kinetics for nickel silicide on Si(1 1 0) substrate is also discussed in this paper.     

Titanium buffer layer for improved field emission of CNT based cold cathode

Carbon nanotube (CNT) based cold cathodes are considered to be the most promising material for fabrication of next generation high-performance flat panel displays and vacuum microelectronic devices. Adhesion of CNTs with the substrate and the contact resistance between them are two of the important issues to be addressed in CNT based field emission (FE) devices. Here in this work, a buffer layer of titanium (Ti) is deposited prior to the catalyst deposition and the growth was carried out using chemical vapor deposition (CVD) technique. There was significant increase in emission current density from 10 mA/cm² to 30 mA/cm² at the field of 4 V/μm by the use of titanium buffer layer due to much less dense growth of CNTs of smaller diameter. Field emission results suggest that the adhesion of the CNTs to the substrate has improved. The titanium buffer layer has also lowered the contact resistance between the CNTs and the substrate because of which a stable emission of 30 mA for a longer duration was obtained.     

Effects of microwave plasma treatment on the field emission properties of printed carbon nanotubes/Ag nano-particles films

The effects of Ar microwave plasma treatment on field emission properties of the printed carbon nanotubes (CNTs) cathode films using Ag nano-particles as binder were investigated. The field emission J-E characteristics were measured at varied plasma treatment time. Significant improvement in emission current density, emission stability and uniformity were achieved for the Ar treated CNTs films, even though the plasma treatment increased the turn on electric field slightly. High-resolution transmission electron microscopy (HRTEM) and Raman spectroscopy revealed the microstructural changes of CNTs after the plasma treatment. The improved field emission properties of CNTs film can be attributed to the generation of a high density of structural defects after treatment, which increased greatly the possible emission active sites. Besides, the formation of the sharpened and open-ended CNTs tips is all helpful for improving the field emission properties of the treated CNTs.     

Hot filament CVD of Fe-Cr catalyst for thermal CVD carbon nanotube growth from liquid petroleum gas

A hot filament chemical vapor deposition (HFCVD) method was used to prepare Fe-Cr thin film on Si substrate. The produced layers were used as catalysts for growing carbon nanotubes (CNTs) from liquid petroleum gas (LPG) at 825 °C by thermal CVD (TCVD) method. To characterize the obtained catalysts or CNTs, X-ray Photoelectron Spectroscopy (XPS), Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM) and Raman spectroscopy were used. CNTs were grown on HFCVD derived Fe-Cr catalyst with the LPG as carbon source successfully. It was found that an annealing process on catalysts enhances the surface concentration of Cr atoms and reduces the sizes of catalyst particles. The grown CNTs on annealed sample were morphologically denser with smaller diameters compared to the as deposited one. In addition, the effect of filament temperature on CNTs was investigated. By increasing the filament temperature from 850 to 1050 °C the surface density and diameters of CNTs were improved.     

Improved emission stability of HfC-coated carbon nanotubes field emitters

Plasma-enhanced chemical vapor deposition (PECVD) method was employed to synthesize the Fe-catalyzed carbon nanotubes (CNTs). Hf films were deposited onto the synthesized CNTs, followed by heat treatment at 1200 °C which could form HfC. Field emission properties indicate that the HfC-coated CNTs have good emission current density due to low work function of HfC and also keep stable emission characteristics under poor vacuum owing to the chemical inertness of HfC. Consequently, field emission characteristics of the CNTs can be improved by the HfC-coated surface treatment compared with the synthesized CNTs.     

Study on changes in the structure of HfSiO and HfSiON dielectrics with different annealing temperature by photoelectron spectroscopy

We have investigated changes of the structure for HfSiO and HfSiON film with different annealing temperature by photoelectron spectroscopy. Core level photoelectron spectra have revealed the mechanism of metallization reaction at the bottom interface between the HfSiO(N) film and Si substrate under vacuum annealing. Silicidation action occurs by annealing at 850 and 900 °C for HfSiO and HfSiON film, respectively, indicating the incorporation of nitrogen enhances the thermal stability. By annealing at 900 °C, metallization reaction is rapidly promoted for the HfSiO film. For HfSiON film, Hf-nitride clusters or Hf-nitride layer and metal-silicide are formed at the bottom and upper interface, respectively, upon annealing at 950 °C.     

A comparative study of field-emission from different one dimensional carbon nanostructures synthesized via thermal CVD system

Different one dimensional (1D) carbon nanostructures, such as carbon nanonoodles (CNNs), carbon nanospikes (CNSs) and carbon nanotubes (CNTs) have been synthesized via thermal chemical vapour deposition (TCVD) technique. The different 1D morphologies were synthesized by varying the substrate material and the deposition conditions. The as-prepared samples were characterized by X-ray diffraction (XRD), field emission scanning electron microscope (FESEM) and transmission electron microscope (TEM). FESEM and TEM images showed that the diameters of the CNNs and CNTs were ∼40 nm while the diameters of the CNSs were around 100 nm. Field emission studies of the as-prepared samples showed that CNSs to be a better field emitter than CNNs, whereas CNTs are the best among the three producing large emission current. The variation of field emission properties with inter-electrode distance has been studied in detail. Also the time dependent field emission studies of all the nanostructures have been carried out.     

Study of Ni/Si(1 0 0) solid-state reaction with Al addition

The characteristics of Ni/Si(1 0 0) solid-state reaction with Al addition (Ni/Al/Si(1 0 0), Ni/Al/Ni/Si(1 0 0) and Al/Ni/Si(1 0 0)) is studied. Ni and Al films were deposited on Si(1 0 0) substrate by ion beam sputtering. The solid-state reaction between metal films and Si was performed by rapid thermal annealing. The sheet resistance of the formed silicide film was measured by four-point probe method. The X-ray diffraction (XRD) was employed to detect the phases in the silicide film. The Auger electron spectroscopy was applied to reveal the element profiles in depth. The influence of Al addition on the Schottky barrier heights of the formed silicide/Si diodes was investigated by current-voltage measurements. The experimental results show that NiSi forms even with the addition of Al, although the formation temperature correspondingly changes. It is revealed that Ni silicidation is accompanied with Al diffusion in Ni film toward the film top surface and Al is the dominant diffusion species in Ni/Al system. However, no Ni_xAl_y phase is detected in the films and no significant Schottky barrier height modulation by the addition of Al is observed.     

Growth and field emission properties of globe-like diamond microcrystalline-aggregate

The globe-like diamond microcrystalline-aggregates were fabricated by microwave plasma chemical vapor deposition (MPCVD) method. The ceramic with a Ti mental layer was used as substrate. The fabricated diamond was evaluated by Raman scattering spectroscopy, X-ray diffraction spectrum (XRD), and scanning electron microscope (SEM). The field emission properties were tested by using a diode structure in a vacuum. A phosphor-coated indium tin oxide (ITO) anode was used for observing and characterizing the field emission. It was found that the globe-like diamond microcrystalline-aggregates exhibited good electron emission properties. The turn-on field was only 0.55 V/μm, and emission current density as high as 11 mA/cm² was obtained under an applied field of 2.9 V/μm for the first operation. The growth mechanism and field emission properties of the globe-like diamond microcrystalline-aggregates are discussed relating to microstructure and electrical conductivity.     

Characteristics of Co-filled carbon nanotubes

The Co-filled carbon nanotubes (CNTs) film was produced on silicon substrate by electron cyclotron resonance microwave plasma chemical vapor deposition (ECR-CVD). The effects of different plasma powers of 200, 300, 400 and 500 W, on the morphology, structure and electrical properties of the CNTs film, were studied. The results showed that the surface density of the vertical nanotubes decreased when the plasma power was higher than 200 W. When plasma power of 300 W was used, the ends of the metal-filled carbon nanotubes (MF-CNTs) became straighter and more uniform. The Co-filled CNTs grown at 300 and 400 W had a current discharge at the applied voltages of 30 and 40 V, respectively. In addition, the surface morphology and the structure of the CNTs film were examined using scanning electron microscopy (SEM) and high-resolution field emission gun transmission electron microscopy (TEM). Energy dispersive X-ray spectroscopy (EDXS) analyses were performed to identify the composition of the material inside the CNTs.     

Titanium-induced germanium nanocones synthesized by vacuum electron-beam evaporation: growth mechanism and morphology evolution

High-vacuum electron-beam evaporation method is used for large area, metal-nucleated germanium (Ge) nanodots and nanocones on Si₃N₄/Si preparation. Nanodot and nanocone arrays with uniform size in bulk-quantity are synthesized using titanium (Ti) nanocrystals as nucleating center at 750 °C with different Ge deposition amount, respectively. The morphology evolution from nanodot to nanocone is studied by atomic force microscopy (AFM). The structure of the prepared sample is characterized by X-ray diffraction (XRD) and Raman scattering. Ge nanocones formed by this convenient fabrication process could have potential applications on nanoelectronics and vacuum electron field emission.     

Effect of substrate and annealing on the structural and optical properties of ZnO:Al films

ZnO:Al thin films with c-axis preferred orientation were deposited on glass and Si substrates using RF magnetron sputtering technique. The effect of substrate on the structural and optical properties of ZnO:Al films were investigated. The results showed a strong blue peak from glass-substrate ZnO:Al film whose intensity became weak when deposited on Si substrate. However, the full width at half maxima (FWHM) of the Si-substrate ZnO:Al (0 0 2) peaks decreased evidently and the grain size increased. Finally, we discussed the influence of annealing temperature on the structural and optical properties of Si-substrate ZnO:Al films. After annealing, the crystal quality of Si-substrate ZnO:Al thin films was markedly improved and the intensity of blue peak (∼445 nm) increased noticeably. This observation may indicate that the visible emission properties of the ZnO:Al films are dependent more on the film crystallinity than on the film stoichiometry.     

Synthesis of silicon carbide films by combined implantation with sputtering techniques

Silicon carbide (SiC) films were synthesized by combined metal vapor vacuum arc (MEVVA) ion implantation with ion beam assisted deposition (IBAD) techniques. Carbon ions with 40 keV energy were implanted into Si(1 0 0) substrates at ion fluence of 5 × 10¹⁶ ions/cm². Then silicon and carbon atoms were co-sputtered on the Si(1 0 0) substrate surface, at the same time the samples underwent assistant Ar-ion irradiation at 20 keV energy. A group of samples with substrate temperatures ranging from 400 to 600 °C were used to analyze the effect of temperature on formation of the SiC film. Influence of the assistant Ar-ion irradiation was also investigated. The structure, morphology and mechanical properties of the deposited films were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and nanoindentation, respectively. The bond configurations were obtained from IR absorption and Raman spectroscopy. The experimental results indicate that microcrystalline SiC films were synthesized at 600 °C. The substrate temperature and assistant Ar-ion irradiation played a key role in the process. The assistant Ar-ion irradiation also helps increasing the nanohardness and bulk modulus of the SiC films. The best values of nanohardness and bulk modulus were 24.1 and 282.6 GPa, respectively.     

热处理对纳米金刚石涂层场发射性能的影响

用旋涂法在金属钛衬底上涂敷纳米金刚石,经过适当的热处理形成金刚石涂层与金属钛衬底的化学键合,即形成衬底与涂层之间的过渡层,从而为纳米金刚石颗粒提供电子,使其成为有效的发射体。用扫描电镜、原子力显微镜、X射线衍射和拉曼散射等手段分析了温度对键合效果以及场发射性能的影响,温度过高或过低都不利于提高纳米金刚石涂层的场发射性能,只有在700℃左右对样品进行热处理,才能得到较好的键合状态。改变涂膜时旋涂的次数以获得不同涂层厚度的样品,对其在700℃的相同温度下进行热处理,发现涂层过厚或过薄都不利于样品发射性能的提高。旋涂9次并于700℃热处理的样品具有较好的场发射性能,其发射阈值场强可达4.6V/μm,而15.3V/μm场强下的电流密度为59.7μA/cm2。     

Investigation of Ni/Ta contacts on 4H silicon carbide upon thermal annealing

Nickel and Tantalum thin films with 3:5 thickness ratios were deposited in succession onto 4H-SiC substrate at room temperature. The samples were then heated in situ in vacuum at 650, 800 or 950 °C for 30 min. Glancing angle X-ray diffraction (XRD), Auger electron spectroscopy (AES) and current-voltage (I-V) technique were used for characterising the interfacial reactions and electrical properties. Amorphous Ni-Ta can be formed by solid-state reaction at 650 °C. The minor dissolved Ni in the Ta metal promotes the reaction between Ta and SiC. With increasing annealing temperature up to 950 °C, the dominant carbide changes from Ta₂C to TaC and a layer structure is developed. Electrical measurements show that ohmic contact is formed after annealing at or above 800 °C.